The present invention pertains to microlithography (projection-transfer of a pattern from a mask or reticle to a suitable substrate) performed using a charged particle beam. Microlithography is a key technology used in the manufacture of semiconductor integrated circuits, displays, and the like. More specifically, the invention pertains to performing alignments of the reticle and the substrate.
A charged-particle-beam (CPB) microlithography apparatus can be used to projection-transfer any of various patterns, defined by a reticle, onto a substrate. As used herein, a charged particle beam is an electron beam or an ion beam. Suitable substrates include semiconductor wafers as well as plates, masks, and reticles.
When exposing a pattern onto a substrate, a suitable alignment reference (for ensuring accurate alignment of the reticle with the substrate, for example) is the position of an alignment mark on the substrate. An alignment mark generally is defined by a layer of a xe2x80x9cheavyxe2x80x9d metal applied to the surface of the substrate or by forming a pattern of depressions in the substrate surface. The position of an alignment mark can be different on various substrates, and the shape or profile of an alignment mark can vary from one substrate to the next, depending upon the particular wafer-processing step.
In CPB microlithography, alignment-mark positions can be measured prior to exposure or during exposure of the substrate with a reticle pattern. Measurements typically are performed using a measurement device in the microlithography apparatus itself. Of course, maximal detection accuracy of mark position is desired.
Various conventional devices are known for measuring alignment-mark positions, including systems that respond to a threshold level of a backscattered-electron (BSE) detector signal. With such a system, the threshold (xe2x80x9cslicexe2x80x9d) level can be set by the operator of the apparatus. A mark position is detected by scanning the charged particle beam across the mark. As the scanning beam passes over a first edge of a mark, the corresponding BSE signal (initially below the threshold level) rises above the threshold level. The signal remains above the threshold level as the beam passes across the mark. As the scanning beam passes over an opposing second edge of the mark, the corresponding BSE signal returns to a sub-threshold level. The system determines the actual mark position by calculating the sum of the first and second locations and dividing the sum by 2.
The most recent CPB microlithography apparatus provide several types of devices for measuring alignment marks. However, the operator must make a selection as to which device to use under a particular condition. This operator-based scheme is subject to error, and hence can result in an inaccurate measurement under the prevailing conditions.
In view of the shortcomings of conventional apparatus and methods as summarized above, an object of the present invention is to provide charged-particle-beam (CPB) microlithography systems capable of performing alignment-mark measurements by any of multiple candidate techniques and that automatically select the optimal measurement technique for the prevailing conditions.
To such end and according to a first aspect of the invention, CPB microlithography apparatus are provided that include an illumination-optical system, a projection-optical system, and a system for detecting a position of an alignment mark. An exemplary embodiment of such a system comprises a backscattered-electron (BSE) detector, a detection-system selector, and a controller. The BSE detector is situated and configured to detect electrons backscattered from an irradiated alignment mark and output a corresponding BSE-data signal based on a quantity of backscattered electrons detected by the BSE detector. The detection-system selector is situated to receive the BSE-data signal and is configured to select, from among multiple candidate techniques for determining the position of the alignment mark, a particular technique based on a characteristic of the BSE-data signal. The controller is configured to perform any of the candidate techniques. The controller is connected to the detection-system selector and is configured to calculate, according to the selected technique and from the BSE-data signal, the position of the alignment mark.
By way of example, the detection-system selector is configured to select the particular technique based on a detected quantity of data in the BSE-data signal, a prevailing exposure condition, or on a detected symmetry of a waveform of the BSE-data signal.
The various candidate techniques can include a slice-level technique in which the position of the alignment mark is determined to be a center of a range of the BSE-data having a value larger than a value corresponding to a selected slice level. Alternatively or in addition, the candidate techniques can include one or both of a autocorrelation technique in which the position of the alignment mark is determined as a maximum value of an autocorrelation function of the BSE data, and a cross-correlation technique in which the position of the alignment mark is determined as a position at a maximum value of a cross-correlation function of reference data and the BSE data.
To facilitate any of the candidate techniques, a wafer can comprise multiple alignment marks each having a respective set of reference data. The reference data can comprise BSE reference data concerning backscattered electrons produced from a reference alignment mark. Alternatively or in addition, the reference data can comprise BSE reference data concerning backscattered electrons produced from a reference alignment mark on a reference wafer, and/or BSE reference data obtained from a simulation of backscattered electrons from an ideal reference mark on an ideal reference wafer.
The invention also encompasses methods for manufacturing a semiconductor device, wherein the methods include exposing a pattern onto a wafer (or other substrate) using a CPB microlithography apparatus as described herein.
The foregoing and additional features and advantages of the invention will be more readily understood with reference to the following detailed description, which proceeds with reference to the accompanying drawings.